What's the highest mileage M278 engine achieve?
 

Just curious if I should be selling my vehicle or not.

M278 engine is wonderful but reading all the stories of all the issues has me fairly concerned it will not last past 150,000 miles unless there's cases that folks can inform the community on without having major issues like the leaking sensors, coolant mixing with the oil, etc.

Going to be hard to say. It will definitely require more maintenance than say, a 3UZ from a Lexus LS430. But they can last. The leaky wiring harness and cylinder scoring are the big detractors. Depending on your specific car either one of those can total you out. However I’ve been looking at W222 S550’s lately and inevitably come across some 12-13 W221’s with M278 engines. I’ve seen a lot with 170k-190k miles and in one instances I found a (supposedly) running 275k example. Hell, I just sold my 2012 S550 with 133k and it was running like a top. The reward is great as the engine is fantastic, but you risk those big issues I meantimes before.

I’m currently at 112K for a 2012 W207. I love the color, prestine interior, and heads turning from many M157, N63, or other more expensive coupe drivers.

However now entering space which could be dangerous. I have replaced all the plastic coolant lines two years ago, and topped off my coolant recently. Keeping a careful eye, along with my indy Benz mechanic, on all things that could cause it to become a problematic engine such as oil leaks and coolant leaks.

All my electronic shocks, control arms, sway bar links, brake pads, and rotors are new with less than 10K miles so my coupe is ready to roll, just don’t want to be wiped out by an engine, engine mounts, or transmission issue. This car was meant to be low maintenance specific to those subsystems but in past year so many articles and mechanic videos on YouTube explaining the failures of the M278 engine have me considering whether to keep or not. I’d prefer to keep as I love the elegance and smoothness of the vehicle.

I like to think you’ll be ok. Keep an eye for oil in the cam position sensors and magnets. There are sacrificial harnesses that are available now that you can use for the position sensors that will add a layer of protection. If you really want piece of mind you can have the engine boroscoped to see if you have any cylinder scoring, but usually the issue pops up early in the M278 life cycle. I think if it’s made it this far you’re likely ok.

Engine mounts are basically a wear item and with 4MATIC I can see it being a concern.

Thanks. Being that it’s a W207 coupe, it’s RWD.

These two videos do irk me what could go wrong…

Gotcha. I was mixing up the sedan and coupe as I believe the W212 sedan only comes in 4MATIC in which case the engine mounts are a pain. If your W207 is RWD then I'd like to think that engine mounts would be a relatively easy job and wouldn't break the bank. Those videos are concerning. Do you have a rattle on startup?

Just FYI Mercedes issued a warranty extension on M278 cylinder heads for 12 years/120k miles. If you ran into misfire issues you could potentially have the heads replaced at a dealer free of charge.

No rattle on startup…smooth and sounds wonderful.

This is what I’m working with:

https://mbworld.org/forums/e-class-c...cessories.html

https://mbworld.org/forums/e-class-c...ting-e550.html

So far 408.500 km., orginal engine - never been opened.

Awesome! How have you done your maintenance schedule? Any regular intervals and special ways to check the health of the engine?

I’m at 124k miles / 200k km and want to get to at least another 150k km.

I had to do replacement of all the plastic coolant line parts and I get regular oil changes. I have checked regularly oil that it’s not seeping into the ECU wiring harness. I do have an unusual issue with small amount of coolant reducing so I may have a small leak in the system somewhere. I just hope it’s not seeping into any internal part of the engine as I have watched videos on YouTube that coolant and oil can end up mixing if the gaskets fail.

small slow leak...
 

check the front radiator CRIMPS all around for leaks - Its a good idea not to extend the life of cheap radiator because of killer internal mayo-mixing issue.

Amazin' radiator leaks both internally and externally. It's a lot easier to replace than a heater-core :D

Timing cover seal... end of life extension
 

This failed coolant seal is between Bank2 engine head and front timing cover.


> CANCELING ROOT CAUSE:
This location sees the hottest oil and the hottest coolant... can we reasonably says that it is affected by extreme heat soaks?

The weak seal destroying the engine is not the cause but a consequence. Replacing seal EARLY only moves the burning heat issue over to the next weakest seal.

What if we could curb these conditions back to nomal 200F heat without any extremes ??

The extreme heat source is the limited pistons cooling to "save gas".
Check out this thread about normal oil pressure....


The cheap item that can help protect engine life is having a working coolant CAP that reliefs coolant pressure without straining the weakest engine seals.

:zoom:

mine lasted 137,000 b4 cylinders 5,6 were scored.
 

2012 E550 coupe, babied it. 6K oil changes. Always Liquid Molly. 45K mi oil through the solenoids into wiring harness. 80k mi blown driver side turbo, dropped the motor to replaced both. 137k mi flashing CEL car stuttering, leak down compression showed some minor loss on cylinder 5 and 6. Scoping confirmed cylinder wall scoring. Done. Not including consumable costs such as radiator, alternator, steering lock solenoid (which is no longer available. had to get mine redone electrically). Car is still pristine so debating the whole rebore\sleeve option or just ditch it.